The increasing popularity of the compact disk (CD) as an audio recording medium, on the tracks of which pieces of music, after being digitized, are successively stored one bit at a time, has also raised the question of broadcasting radio transmissions digitally, in other words of modulating the carrier associated with the transmitter by using binary signals.
In seeking answers to this question, the influence upon a digital signal of propagation problems of radio transmission carriers, and above all of radio reception at various locations by a car radio installed in a vehicle, must be properly taken into account.
The signal strength of a carrier at a particular location is determined both by the distance from the transmitter and by multipath reception capabilities; these capabilities can vary greatly, even over short distances. The resultant fluctuations in signal strength for car radio reception of the transmitter may be great enough to produce a momentary loss of receiving capability. In binary signal transmission, this means that bit streams of variable length will be missing, which is perceived as a major disturbance, although in analog signal transmission such an interruption would be perceived as only minor interference. The disturbance in reception is also frequency-dependent, because of the multipath characteristics.